An ongoing photo essay documenting the replacement of the Sarah Mildred Long Bridge
If you've driven down Market Street in Portsmouth past the State Pier in the last few weeks, you have probably noticed the large yellow structure being assembled. This is a set of forms that will be used to cast segments of the four towers on the bridge.
In the rendering below, I've outlined one of these segments. There are 80 of these in all. The segments are hollow to allow the counterweights to drop within.
In the photo at the top, and in the one below, you can see workers assembling the internal section of the mold for the first segment. Part of the external form is also visible on the right in the below photo. The second photo below is of some of the other external form sections.
Once these forms are in place, a rebar cage that has been pre-assembled on the ground will be lifted into place. This will sit outside of these forms with a minimum clearance of 3 inches between the form and the steel. The rest of the external forms will then be assembled around it, and concrete poured. Since so many of these cages will be required, jigs have been constructed of welded angle steel. Below, you can see the rebar cage still sitting on the jig. This particular cage has a large void in it (at the left of the photo) where a door will go. This is the at the bottom of the tower, and will allow access to the machine rooms. The second photo shows the lifting frame that will be used to hoist the rebar cages into place.
Once the initial tower segment has been cast and cured, it will be transferred to the other half of the structure. Here's why the structure is so tall: the next segment of the tower will be cast directly on top of the previous segment. Below is a photo of the setup that allows for this. Remember that the segments are hollow. The completed segment will be lowered onto the pads seen at each of the four corners about a quarter of the way up from the bottom of the photo. All of the structure above that level will be inside of the completed segment. The top of that segment will be flush with the platform at the top of the photo. The forms for the next segment can then be assembled with the completed segment serving as the bottom form, and the new segment cast such that it mates perfectly with the one below it. This process is known as "match casting". Once the new segment cures, the previous one will be set aside, the new one lowered into this position, and the next one cast on top of it. And so on, until each tower is complete.
There will be tubes cast into each segment that extend from the base to the top of the tower. These will house post-tensioning continuity tendons - essentially steel rods that will be tensioned after they are inserted in order to hold all of the tower segments together. Like a truss rod on a guitar, if that means anything to you. The plastic tube in the cage below will form the void for one of these tendons.
Finally, as each segment is ready to be moved to the tower position, it will be carefully transferred to this creeper. This will allow it to be moved to a barge, and floated to the position of the tower. Remember that each segment is cast on top of the previous one, so it would very difficult to replace one if damaged. This creeper keeps the platform very steady as it moves over uneven terrain.